Interested in expanding your DS library... while it's a little difficult to tailor my recommendations, here's a list of games to consider picking up...
* 999: 9 Persons, 9 Hours, 9 Doors -- 1st title in the "Zero Escape" saga.
* Phoenix Wright: Ace Attorney series (4 titles)
>>> Apollo Justice: Ace Attorney -- 4th in the Phoenix Wright series
* Professor Layton series (4 titles)
* Dragon Quest IV, V, VI & IX (4 titles)
* Final Fantasy III / IV (2 titles)
* Chrono Trigger DS
* Radiant Historia -- A hidden RPG gem on the system
* New Super Mario Bros.
* Tetris DS
* Retro Game Challenge -- The only US-localized release of the "Game Center CX" series
* Puzzle Planet League
* Legend of Zelda: Phantom Hourglass / Spirit Tracks (2 titles)
* Warioware: Touched! / DIY (2 titles)
* Pokemon series (4th - 5.5th Gen)
* Animal Crossing: Wild World -- Better w/ Friends (lack of online support w/ NWC shutdown kills some appeal)
* Clubhouse games -- Better w/ Friends, but still good solo (sometimes, you just want to play an old parlor game)
There's bound to be some titles that I'm forgetting, but I'm omitting titles that you already have & this is a pretty good "Greatest Hits" list to check out. I hope this helps!